1. Field of the Invention
The present invention relates to a corona discharge device used in an electrophotographic image forming apparatus such as a copying machine and a printer.
2. Description of the Related Art
In many electrophotographic image forming apparatuses, corona discharge devices have been used, for example, as a charger for uniformly charging a surface of an electrostatic latent image carrier such as a photosensitive drum prior to formation of an electrostatic latent image corresponding to an original image, a transfer charger for transferring a toner image formed by development of the electrostatic latent image onto a transfer sheet of paper, and a separation charger for separating the transfer sheet from the electrostatic latent image carrier after the transfer of the toner image.
The corona discharge device generally employs a discharge electrode formed of a wire electrode which extends continuously along a charge receiving member, i.e., a member to be charged.
Recently, it has been proposed to use a DC (direct current) charge device as a discharge device employing a saw-toothed electrode instead of the wire electrode, for example, in Japanese Laid-Open Patent Publication No. 5-19591 (1993). Further, a discharge device provided with a charging roller has also been proposed.
If an organic photosensitive member is used as the electrostatic latent image carrier, the surface of the photosensitive member is negatively charged. If a well-known selenium photosensitive member is used, the surface is positively charged. In this manner, the member to be charged such as the photosensitive member is charged to attain an intended polarity. For this purpose, the DC corona discharge has generally been used.
According to the corona discharge device employing the wire electrode, however, a majority of a high voltage energy, which is applied to the wire for the discharging operation, is consumed to generate ozone, so that a large amount of ozone disadvantageously generates during the discharging operation.
If the ozone concentration is high, product such as NO.sub.X which is formed by oxidation with ozone adheres onto the surface of the electrostatic latent image carrier such as a photosensitive drum, so that the electric resistance at the surface of the electrostatic latent image carrier decreases, resulting in movement of the electric charges (electrostatic latent image), i.e., a so-called a "dislocation of the image".
In the prior art, it has been attempted to exclude the discharge product such as the aforementioned product and ozone, from the image forming apparatus, by providing a ventilation fan and/or an ozone filter. Such discharge product adversely affects the human body, so that the discharge product which is released from the apparatus has been regarded as a serious issue in accordance with a movement for the protection of the environment. In particular, the amount of generated ozone released from the copying machines and others has been restricted in accordance with the approval standards relating to a blue angle mark for indication of safety in Japan and European countries as well as the UL standards which are the safety standards in U.S.A.
The DC corona discharge device having the saw-toothed electrode described above can reduce the amount of generated ozone to a value from about 1/3 to about 1/4 of that by the device having the wire electrode. Even if the discharge device including the saw-toothed electrode is employed, a relatively large amount of ozone is released from the image forming apparatus as a whole, because the image forming apparatus includes, in addition to the discharge device for charging the electrostatic latent image carrier, the transfer device and others which perform the discharge operation.
Further, the discharge device employing an electrode having needle-like tip ends such as the saw-toothed electrode does not have sufficient reliability. If the discharge operation is performed for a long time, tip ends of the electrode are oxidized, and dust or the like is deposited the tip ends, resulting in irregular discharge.
Further, the electrode having needle-like tip ends such as a saw-toothed electrode applies the electric charges through a plurality of discharge points to the charge receiving member. It is therefore necessary to set an appropriate pitch between the discharge points as well as a distance between the discharge point and the charge receiving member in order to apply the electric charges uniformly to the charge receiving member.
If the pitch between the discharge points is excessively small, interference between electric fields by adjacent discharge points occurs, resulting in the irregular discharge. If the pitch is excessively large, a large difference occurs in the discharge voltage between a position near the discharge point and a position remote therefrom, resulting in the irregular discharge. If the space or gap between the discharge point and the charge receiving member is excessively small, the electric charges are applied to local portions of the charge receiving member, resulting in the irregular discharge. If the gap is excessively large, the supply voltage for the discharge must be large, resulting in increase of the sizes of the apparatus.
Although the discharge device employing the charging roller described above generates a smaller amount of ozone than the discharge device employing the wire electrode, it cannot comply with the high-speed image formation which has been required in recent years.